Spectrophotometer and spectrophotometric analysis and prediction



Oct. 10, 1939. 2,176,013

SPEGTROPHOTOMETER AND sPECTROPHOTOMETRIcA ANALYSIS AND PREDICTION o. w. PINI-:o

Fil-ed Aug. 12, 1937 5y Sheets-Sheet 1 INVENT OR.

Oc't. 10, 1939. o. w. PlNEo Filed Aug. l2, 1937 3 Sheets-Sheet 2 ATTORNEY.

Oct. 10, 1939. Q w P|NE0 Y 2,176,013

SPECTROPOTOMETER AND SPECTROPHOTOMETRIC ANALYSIS AND PREDICTION Filed Aug. l2, 1937 3 SheetS-Sheel 5 ATTORNEY.

Patented Oct. 10, 1939 UNITED STATES PATENT oFFlcE SPECTROPHOTOMETER AND SPECTROPHO- TOMCETRIC ANALYSIS AND PREDICTION Application August 12, 1937, serial No. 158,821

4Claims.

This invention relates tp recording spectrophotometers and more particularly to recording spectrophotometers of the flickering beam type.

A number of spectrophotometers have been 5 designed using flickering beams for measuring reflectance or transmission of standards and samples. Such flickering beam photometers usually incorporate a polarizing element such as a Nicol or Rochon prism as a photometering elel ment through which the light of the flickering beams passes. An examplepf such a photometer is the one described in my prior PatentrNo. 2,107,836 dated February 8, 1938. Photometering is effected by rotating the polarizing element. l However, the angle through which the photometering prism has to be turned to compensate for a given unbalance of reflection o1' transmission between sample and standard at a given Wavelength bears no linear relation to the ratio 20 of the reflection or transmission of sample and standard.

It has been proposed to read a pointer linearly driven from the photometering element. If such a pointer were to be associated with a recording g5, surface so as to draw a curve, readings thus obtained would not be directly translatable into ratio of reection or transmission, because the ratio varies as the square of the tangent of the angle through which the photometering element 30 is turned to effect balance. s

In my prior patent above referred to, I have described a method of driving an indicating or recording element in which the disadvantages ini, herent in the spectrophotometers of th type 35 Where an indicating or recording element, is linearly driven from the photometering element are avoided by interposing a drive linkage between the photometering element and the indicating device, Whether Visual or recordingwhich 40 linkage includes a drive of varying ratio, the ratio variation being such that the motion of the indicating device is proportional to the square of the tangent of the angle (tan2 a). As a result,

-. indications are obtained or records are made l5 which are proportional to the reflection or transmission ratio. With the above described drive, therefore,` it is not necessary to calculate from visual or written indications of the spectrophotometer to obtain the correct readings.V Thus, for

50 example, with a recording spectrophotometer described in my patent above referred to having a linkage between photometering element and recording device containing a'variable ratio drive l. varying irr accordance with the tangent squared 55 function referred to above, or a linear multiple thereof, a curve will bev drawn, the ordinates of whichcorrespond directly to reflection or transmission ratio and actual figures can be taken from such a curve if it is drawn on ordinary coordinate paper of suitable scale. 5

There are, however, a number of cases where response which is directly proportional to the transmission or reflectance ratio isnot satisfactory. The most common case where the simple recording spectrophotometer of my prior patent will not give directly all the data needed is when transmission or reflectance samples of varying color concentration are encountered and particularly Where a sample is of unknown color concentration as occurs when attempts are made to analyze a colored sample without knowing the strength of dyeing. When drawn on this simple type of recording spectrophotometer, the curves for two samples of identical color but of diiferent color concentration will not be of the same shape because the variation with concentration is not a linear one. In the case of transmission samples the variations may be in concentration of color in the sample or in length of cell.

'I'he difiiculties encountered with samples oli-25 varying color concentration are overcome in the' present invention in which the drive of varying ratio contains the tangent squared function modified by a second function which translates nonadditive variations due to differences in concentration or light path length into additive variations so that curves for different concentrations of the same color or same mixture of colors will Toe of the same shape and will merely be displaced vertically on the coordinate paper by an amount 35 proportional to the logarithm of the concentration.

The non-additive effects of varying concentrations can be transformed into additive effects because transmission is a function containing as 40' one of its elements the specicvabsorbing property of the colored medium whichat every wave length is a specific property of the color and is invariant with concentration orlength of paththrough the medium. The transmission follows the formula ci roi-fm Where the square symbol l defines the specific absorbing property of the color and L is length of light path. For a single color the specic absorbing property is a function'of wave length, 15100 multiplied bythe concentration c1 so that ekl) OIL Taking the natural logarithm of the reciprocal l of the transmission, the equation is transformed as follows:

If the logarithm of both sides of the equation is taken, the following results.

log log t-(15=log R100-Hog cpi-log L urements. Log c1 varies with each concentration 1 but is normally constant for the measurement of any sample. Therefore the only factor on the right hand side of the equation which is a variable function and which will therefore determine the shape of the curve drawn is'the specific absorbing property of the color itself and changes in concentration will merely displace the curve vertically on coordinate paper. Therefore, for transmission a drive of varying ratio, the variation being the tangent squared function modified by the function on the left hand side of the equation above given, will draw a curve the shape of which depends only on the specic absorbing property of the dye and is invariant with concentration. Of course, in order to read numerical values from the curve, it must not be drawn on ordinary coordinate paper, but on paper the ordinates of which correspond to 1 log log E The above derivation of the formula for determining the variation of ratio in the drive presupposes only a single color with a single absorbing property IciOr). Frequently measurements have to be made on a mixture of colors. I have found that in\general each color molecule behaves practically as if the other color molecules were not present. In other words, ifv there are two or more different types of color molecules in the solution 'the transmission of which is tobe measured, the square symbol l instead of representing a single kim) times a single concentration, c1, has the following generalized formula 4) 't i=k1o c1+k2 i c2 So long as the relative proportions of molecules in the mixed color remain the same the invariance of the shape of the curve drawn for 1 10g 10g 'im remains, as will be apparent from the following demonstration. Let us assume the generalized formula for l given above and let us .assume that the concentration of each component is doubled. The resulting formula will be 5) klamm-Hao) ze This can be written (6) 2[k1(7\)c1+k20.)c2 .l and when the log is taken, it will be' obviously (7) log [k1( \)c1+k2(i)cz .l-l-log2 In other words', it has simply added an additive term which'will result in displacing the curve vertically in proportion to the logarithm of the change in concentration. So long, therefore, as we are dealing with the same color mixture, that is to say with the same proportion of different colors, the present invention in its more complex form permits the tracing of curves which are invariant in shape with concentration and it is therefore no more necessary to know the actual concentration of a mixture than it is to know the actual concentration of a single color. In bothl cases, the curve will be identical in shape regardless of vdifferent concentrations. The cases where the proportions of different colors in a mixture may vary will be discussed at a later point and in particular with referenceto Figure 6 of the drawings.

`In the type of spectrophotometer described in my prior patent above referred to, t0() is proportional to tanza where a is the angle through which the photometering element is turned. Therefore, in order to draw, on such a machine, a curve which is invariant in shape with concentration, Formula 3 must be modified to read (8) log log cot2a=log 1G10() -l-log c1 -l-log L Instead of subtracting the curve of one dye from the curve of the mixture, the mixture may be measured against the corresponding concentration of all but one of the dyes as a standard. The machine will then draw a difference curve which will be the curve of the specific absorbing power of the remaining dye.

It is an important advantage of recording spectrophotometers of the present invention using the varying ratio drive'to give a curve for thespecific absorptive property o f the color that it4 is not necessary to make measurements forcolor on the particular material of interest at the moment. Thus, forexample, it is entirely possible and very convenient to predict the color which will be obtained on a given sample from standardized data on the dyes obtained for example, by transmission measurements of water solutions.

The formula (1) is suilicient to illustrate the fact that the presence of light absorbing materials in a medium gives rise to a specic absorbing property ofthe medium independent of how such property may be manifested in a measurement, and that the specic absorbing properties of the component materials, each of which is a single characteristic function ofl .wave length or curve on a wave length plot, combine to give the specific absorbing property of a compound medium aiways according to the simple additive law expressed by the square symbol. This law is 55 ,to different measurements made on the medium,

and, in the case of a particular measurement, in away determined by macroscopic, geometrical,

and other considerations of the particular medium and measurement. The problem which the invention sets out to solve is to exhibit the effect of such absorbing materials in concrete cases in such a manner that the effects of the different materials may be segregated with a clarity equal to that of thegeneral mathematical statements of the above two formulae.

The simplest type of drive of varying ratio is a cam having a profile which in conjunction with the cam follower., corresponds to the function of the variable ratio. For practical operation, I

ylo

find that a -cam drive presents important advantages and is the preferred variable ratio drive. However, the invention is in no sense limited to the use of a' cam and any other variable ratio drive with suitable variation can be used, such as, for example, eccentric gearing and the like.

In the drawings, Fig. 1 is a diagrammatic il- -lustration of a flickering beam type of spectrophotometer according to my prior Patent No. 2,107,836 dated February 8, 1938';

Fig. 2 is an elevation of a recording means connected to the spectrophotometer of Fig. 1 and incorporating a cam for recording directlyr percentage variations of transmission or reflectance of sample and standard;

Fig. 3 is a series of curves for different concentrations of the same dye .drawn by an instrument having a simple tangent squared cam as described in Fig. 2;

Fig. 4 is a series of curves for the same concentrations of the same dye as in Fig. 3 but drawn by an instrument in which the simple tangent squared cam of Fig. 2'is replaced by a complex cam to trace directly the curv'e of specific absorptive property;

Fig. 5 shows two curves representing two different component dyes and a curve representing a mixture lof the two components, all applied to a transmisison scale;

Fig. 6 illustrates the superposition of a suitable scale on Fig. 5 toppredict the result of such an addition, the plotbeing shown fragmentarily.

In Fig. l, the monochromator consists of incandescent lamp I, preferably of the low voltage compact filament type, a pair of condensing lenses 2, a variable slit 3, collimating lenses 4, first prism 5, lens 6, mirror l, knife edge 8, second prism III, colllmating lenses II and exit slit I2.- The mirror and knife edge may be reciprocated` by means of the rod I3 moving the slit defined by the knife edge through the spectrum so that the light passing slit I2 can be caused to traverse the visible spectrum.

The light leaving the slit I2 of the monochromator is collected by lens I5 onto the photometric prism i6 which is of a conventional Nicol or Rochon type. The prism is mounted in the hollow shaft Il which is rotated by the -motor I8 which is self-starting, the direction of rotation depending on the phase of the A. C. current energizing the armature as compared to the A. C. current in the field through the worm I9 and ring gear 20. The beam then passes through a bi-refringent plate 5I which rotates with the hollow shaft l but is adjustable thereon. The plate is adjusted to give retardation such that the light leaving the prism i6 is elliptically polarized. The relative size of the minor axis of the elipse is such that at the setting of prism it corresponding to zero reflectance of the sample l23, there is sufficient light in the beam illuminating the standard to balance out surface reflection from the sample. The correction for surface reflectance described above is not claimed as such in the present application.

The plane polarized beam leaves the photometric prism and passes through a Wollaston prism 2i where it isseparated into two beams plane polarizedv at right angles to each other and the two. beams pass through a flicker prism 22 which is mountedin a hollow shaft 23 being the center of the rotor 26 of a synchronous motor, the field 25 of which is energized by 60 cycle a1- ternating current which is also used to energize the field 26 of the motor I8; The beams after lof 10 Watts.

passing through the flicker prism and the decentered lenses 9 enter into the integrating sphere 21 and strike targets 28 and 29. Transmission cells 30 and 3|. (shown in dotted lines) are interposed in the two beams before they enter the integrating sphere.

In the wall of the integrating sphere is an aperture 32 through which the integrated light from the sphere passes and impinges on a photocell 33. The current from the photocell is amplified at flicker frequency by a high gain vacuum tube amplifier 34 and the amplified current which is also at flicker frequency is then fed to the armature of the motor I8 which rotates the photospectrum. The photometric prism plane polarizes a beam defined by the shape of the slit I2 and this beam will vary slightly in intensity with the degree of polarization in the monochromator at`diiferent wave lengths. The plane of polarization of the beam leaving the photometric prism is, of course, determined by the position of the prism which in turn determines the relative brightness of the two polarized beams leavingv cells of material are inserted at 30 and 3I the beams will vary in accordance with they transmission characteristics at the particular. wave measured. The paths of the light rays are indicated on the drawings by lines in the customary manner. y

Any unbalance in total light reflected from the respective targets during a complete cycle will produce a total variation of light in the integrating sphere, the variation being at flicker frequency, that is to say, cycles per second. This 60 cycle current, together with any stray fluctuations at other frequencies which may result from the characteristics of the targets, are then amplifled by the high gain amplifier which should be capable of amplifying a voltage produced by y length of the co-mparison cell and the cell to be the photocell in the grid circuit of -the first amplifled tube of the order of 10 microvolts so as to produce an output from the amplifier of the order This amplified current which, of course, contains as one of its main components the 60 cycle flicker frequency is then fed into the armature of the induction motor I8. Since the ileld of the induction motor It receives pure 60 cycle A. C. the motor will respond substantially only to the 60 cycle flicker frequency; in other words, the motor may be considered as a very -sharply selective device which responds only to the tube of the amplifier and hence the vmotor I8 will stop. The highly selective characteristic of the motor I8 is very important since stray effects make it impossible to produce in the photocell output circuit anything like a pure sinusoidal 60 cycle current. On the contrary an oscillogram of the amplied photocell output shows the presence of anfenormous number of stray frequencies. Because of the great selectivity of the motor I8, however, it is actuated only by 60 cycle component and therefore gives a true response to beam unbalance. The direction in which the motor I8 rotates depends, of course, on the phase of the 60 cycle component in the amplifier output which reverses with the reversal of the relative intensity of the reflected light from the two beams. Thus, for example, if there is unbaiance so that the light reflected from target 28 is greater than that from 29, the resulting 60 cycle component after being amplified will have one phase, as compared to the 60 cycle current flowing inthe field windings orf the motor, whereas if the opposite condition takes place, that is to say, if the reflected light beam from target 29 is greater than that from 28 the 60 cycle in the output will have eX- actly the reverse phase and, of course, will turn the motor I6 but in the opposite direction. Before the photometer is put into operation initially it is necessary to adjust the phase relation of the amplifier and motor I8 so that the motor turns in the right direction and with maximum torque.

This latter is-adjusted by positioning the field 25 with respect to th'e prism 2l. A reversal of leads, of course, will result in the motor turning the photometric prism away from balance instead light for equal movements of the table 35.

of toward balance.

Fig. 2 shows in detail recording means used in conjunction with the spectrophotometer of Fig. 1 to produce a continuous graph of reflection or transmission,characteristics of a given sample at the various wave lengths throughout the spectrum. Like parts bear like numerals. A wave length traversing motor I4 drives a worm 36 which rotates a Worm wheel 3'I keyed on a screw threaded shaft 38, the rotation of which causes a table 35 to move along thelshaft, the table is supported on a suitable ball bearing 39 and is provided with a disengaging button 40 operating a spring held latch 4I which engages the screw thread on the shaft 38. One. end of the table 35 carries a cam 42 which bears on a cam follower 43 on the end of the shaft I3 which shifts the monochromator mirror carriage to cause the monochromator light to traverse the spectrum'. The profile of the cam 42-is chosen so that the movement of the shaft I3 will cause equal displacement of wave legth in the monochromator Since the traversing of the monochromator is directly effected by movement. of the table the latter can be moved rapidly by pressing the button 40 and moving the table by hand. This permits a. rapid return of the table when a graph has been drawn and makes it unnecessary to return the table slowly by operation of the motor I4. Of course, if desired, the motor I4 may drive both the table and the`cam directly; in which case manual return is not possible. I prefer, however, to use the form of drive shown where the table is moved by the motor and the table, in turn, actuates the monochromator.

A stylus`43 is on the threaded shaftv 44 which is driven by the photometric prism motor I8 through a worm I9 and worm gear 20. The shaft 44 also carries a worm gear 45 which the prism I6 is moved. The stylus 43 will therefore respond to the percentage of difference in total light from the samples in the two beams and, therefore, the recording means will draw a graph which shows percentage of light transmission or reflection, as the case may be, of the sample at any wave length. In order to make ne adjustments so that the reflection from or transmission of the standard will correspond to 100% on the graph paper which is clamped to the table 35, it is sometimes necessary to make minute adjustments on the prism I6 and this may be done by means of the set screw 54 which permits shifting the hollow shaft il slightly in the crank arm 49. Such adjustment will be necessary only when the machine is f'lrst put into operation.

It is sometimes desirable to change the percentage transmission scale on the graph, thus,

for example, it may be desirable to cause a greater movement of the stylus 43 for a given percentage change in reflection or transmission characteristic of the sample. fected by changing the gear ratio between the shaft 44 and the shaft 41 or by using a shaft 44 with a different pitch of screw thread. In every case the response of the stylus 43 will still be proportional to the square of the tangent of the angle through which the prism i6 is turned but this proportion may be multiplied by constant factors introduced by the gear ratio and screw pitch.

In operation a sample to be tested for reflection forms one of the targets, for example 29, whereas target 28 is a standard white surface which may be obtained by smoking a magnesium carbonate block with magnesium oxide or in the case of fabrics an undyed fabric may be used as the standard. The synchronous motor rotating the flicker prism 22 is then started and brought up to speed. 'I'he wave length traversing motor I4 is then started at one end of the spectrum. 'I'he unba-lance between the reflected beams at the start is amplified and the motor I8 rotates until the photometric prism I6 has been turned by the cam through a sufficient angle to balance the beams. The amount of rotation will move the stylus down the paper to the ordinate corresponding to a certain percentage of reflection. As the monochromator slowly traverses the spectrum the table 35 moves instep with it and at each wave length the motor `III rotates in one or the other direction until the photometric prism has restored balance in the reflected beams. The stylus therefore draws a continuous This may be efcurve or graph which is a. true plot of the percentage reflection of the given sample corresponding to each wave length of the spectrum. Since cam 46 causes the motion of thestylus to be in proportion to the square of the tangent of vthe angle vthrough which photometric prism I6 is turned the graph will-plot true percentages.

When it is desired to measure the transmission of a colored solution or transparent or trans- F' parent material is inserted at 3|. The measurement of the light transmission at different wave lengths proceeds precisely as described above in connection with the recording of a curve deflning the reiiectingcharacteristics, although, of

A course, the nature of the curve for a given coloring matter will naturally be diierent.

, devices may be used, such as, for example, a

stationary prism with a rotating half wave plate, a Kerr cell and the like, such types being described in my copending application Serial No. 11,600, filed March 18, 1935, now Patent No. 2,126,410 dated August 9, 1938. 'I'he present invention is, of course, not limited to lany particular type of flicker photometer.

On Fig. 3, the curves for the various concentrations of the same dye are shown at a, b, c and d. It will be noted that as the concentrations vary, the shape of the curve varies and unless the concentration is known, nothing very definite can-be determined as to the nature of the dye itself from the examination of any particular curve. The curves of Fig. 3 are, of course, drawn by a recording spectrophotometer using the simple tangent squared cam as illustrated in Fig. 2. If this cam is substituted by one which gives directly for transmission measurement the specic absorptive property, the curves of the same concentrations of the same dye as in Fig. 3 now appear as a', b', c and d on Fig. 4. It will be noted that the shape of these curves is invariant and they are merely displaced vertically by variations in concentration.

0n Fig. 5 are shown curves e and f which represent two different dye components. Curve y,- 'on the same figure, shows the resultant of thev mixture of these two components'all applied to a transmission scale. When curves e andv j' are drawn in terms of specic absorptive property using the complex cam, it is necessary only to add their values read from log scale which may be superimposed over the curves as shown at 52 in Fig. 6. Curve y can be calculated `by reading the antilogs of the two curves e and .f from a suitable log scale shown in the figure, adding them together, andl re-enterlng the logarithm of their result. Mathematically, this is demonstratedas follows:

log[ le=logkce and . log[ lf=1og kie; from Jthis' log [lewlog llcece-l-kfcf] The above method can of course be used with mor than two components.l

It will be noted that the drive of Fig. 2 may be divided into two portions, one linear drive from shaft 41 to pen 43 and a portion having a varying ratio, the single cam 46 and follower 49.

Although the general description of the present invention has been more particularly applied to dyeing and pigment problems, it should be understood that this was purely by way of illustration and the machine of the present invention is gen- -'erally applicable to all transmission problems of and the standardization of their products. It is furthermore contemplated to use the invention for the prediction and analysis of spectrophotometric measurements, even those having no reference to color or even outside the visual range, for instance in chemical analysis and medical diagnosis.

'I'he description of the inventionhas been in terms of its use with spectrophotometric measurements of visible light. vThe principles of the invention are obviously-y however, applicable to any radiant energy capable of measurement by flickering beams. Thus, for example, by using suitable light sources and quartz optics with quartzcalcite polarizing prism together with proper photocell, the measurement can be carried into the ultra-violet and the drive linkage between photometering prism and record elements performs precisely the same functions as in the case of visible light. Similarly, measurements in the i infra-red can be made wherever it is possible to obtain suitable monochromatic bands of infrared light, suitable optics and a suitably responsive photocell which may in the case of longer Wave infra-red light be some type of bolometer. f

Therefore, when the term light is used in the claims, it is not intended to vlimit this term to its narrower meaning of visible light, but both ultraviolet and infra-red light is included.

'I'his applicationis in part a continuation of my 1 copending applications' Serial No. 17,392 filed April 20, 1935, and Serial No. 65,489, filed February 24, y1936, now Patent No. 2,107,836 dated February 8, 1968. I claim: l

1. In a recording spectrophotometer of the flickering beam type for measuring the transmission of a colored sample in which a photometering element is moved to control the relative intensities of the beams and thereby to compensate for the transmission of the colored sample and in which a recording device is actuated to move a recording surface in accordance with wave length or frequencies of monochromatic light being introduced into the spectrophotometer, the improvement which comprises a drive linkage between the photometering element and a recording means operating on the recording surface, said linkage including a drive of varyf ing ratio, the ratio of variation of said drive being suchthat'the movement of the recording element isproportional to where t is the transmission of the sample, wherebyl a curve is drawn on the recording surface which is invariant in shape with variations of concentration or of thickness of the colored samy ple.

2. In a recording spectrophotometer vof theproportional to the square of the tangent of the 3: A recording spectrophotometer according to angle through which the photometering prism is claim 1 in which the drive of varying ratio is a. lrotated modified by a. function such that the cam.

movement of the recording element 1s proportion- 4. A recording spectrophotometer according to a1 to log log cotza, whereby a curve isdrawn claim 2 in which the drive of varying ratio is a 5 which is invariant in shape with variations of cam. concentration or thickness ofthe colored sample. ORRIN WESTON PINEO. 

